Conventional X-ray tubes include a source of a beam of electrons which, under the influence of a high voltage, are caused to impinge on an anode structure. The anode emits X-rays in response to the incident electrons.
It is known to provide multiple focal spots in X-ray tubes in order to generate X-ray beams along a plurality of paths.
One type of multiple-focal spot X-ray tube employs a plurality of independently-controllable heating cathodes. An example of such a system is German Pat. No. 406,067, in which the cathode heating filaments may be supplied by separate heating-current sources, or may be serially connected and selectively supplied with heating current from a common source via a switch. Another example is U.S. Pat. No. 3,452,232, which shows the use of a multiplicity of cathodes, each having a filament element for achieving multiple focal spots.
Another common type of multiple-focal spot system uses a plurality of filament elements with either a single cathode structure or a plurality of cathode structures. Examples of such systems are shown in U.S. Pat. No. 4,315,154, U.S. Pat. No. 4,109,151, and U.S. Pat. No. 3,649,861.
Yet another technique for providing a movable focal spot is shown in United States Pat. No. 4,128,781, which shows an X-ray tube in which the movement of the focus in space is accomplished, without having to move the X-ray tube itself, by moving the cathode along an arcuate path with respect to the anode. A similar system is shown in U.S. Pat. No. 4,072,875, which provides for altering the point of incidence of the electron beam on the anode by moving the anode with respect to the other parts of the tube.
Additionally, a single electron beam could be deflected to selected focal areas to yield a multiple-focal spot system. An example of this is U.S. Pat. No. 4,048,496, which shows an X-ray tube having an electron beam which may be directed to selectable ones of an array of targets to yield X-rays having selected wavelength spectra; the beam in this patent is directed through the use of deflection plates 26. Another example is U.S. Pat. No. 4,229,657, in which an electron beam is deflected to yield a movable impact zone on the target anode. The deflection device uses a magnetic system which employs a rotating magnetic field to cause the emission of photons in several directions, either successively or simultaneously. Finally, U.S. Pat. No. 3,250,916 shows a system in which a single cathode structure produces a single beam of electrons from a single filament, and in which a pair of deflection plates are positioned on opposite sides of the cathode, and are connected by conductive supports to external leads for supplying variable electric potentials to the deflection plates. The potentials on the deflection plates are varied such that a continuous or intermittent beam of electrons from the filament may be alternately switched between two focal spots spaced apart on the target of the anode.
The above-described systems are disadvantageous in that they require a plurality of cathodes, a plurality of filaments, or the use of deflection plates for deflecting the electron beam, thus requiring a relatively large number of structural components.
Moreover, in the systems which use deflection plates, e.g., U.S. Pat. No. 4,048,496 and U.S. Pat. No. 3,250,916, because the deflection plates are located a relatively large distance from the cathode, the electron beam will have achieved appreciable energy by the time the electrons enter the deflection region, and thus, a considerable voltage will be required to achieve deflection of the beam. Also, the necessity of placing the deflection plates between the cathode and anode requires a relatively wide spacing between the cathode and anode, which creates size difficulties in certain applications. Further, the large cathode-to-anode spacing results in a larger-than-normal focal spot size, and this, in turn, requires a complex and costly electron-beam optical system to compensate for this oversized focal spot. In some high-powered applications using rotating anode targets, grid control may become necessary in order to avoid overheating the anode. The above-described systems using separate deflection electrodes have an additional disadvantage in that grid control would require a total of five wires--two for the filament, one for the cathode cup, and two for the deflection electrodes--whereas conventional X-ray tubes and high-voltage components utilize only four wires; thus, a five-wire system would impose additional complexity and expense with respect to the high-voltage cable and connector which deliver power from the high-voltage power supply to the X-ray tube. Also, in U.S. Pat. No. 3,250,916, for example, the invention relates to stereoscopic radiography in which the separation between focal spots corresponds to the distance between the eyes, i.e., the interpupillary distance; although the large cathode-to-anode distance is suitable for such an application, this large distance is unsuitable for other applications where the distance between focal spots must be very small, for example, on the order of 1-2 mm.